Wait I might have misunderstood. Is it a new unique event-item one obtains from completing the ToB, or is it an item from the existing rare droptable? Can anyone confirm?

Everyone gets a nylo pet upon logging in.

When you complete your first raid since the update, you automatically get one of the rare drops are a reward (scythe, rapier, justiciar, hilt, pet, etc). This means that any experienced team can do 3+1 or 4+1 raids with a new account for an automatic rare item every raid.

Today, we have added a countermeasure to prevent players from abusing the first Theatre of Blood completion. First off, you will need to complete five rounds before getting a guaranteed rare item. Secondly, you now need to deal at least a significant portion of the total damage in order to be eligible for your first completion reward:

Today, we have added a countermeasure to prevent players from abusing the first Theatre of Blood completion. First off, you will need to complete five rounds before getting a guaranteed rare item. Secondly, you now need to deal at least a significant portion of the total damage in order to be eligible for your first completion reward: